Insulation for silicon irons



July 18, 1944 C. c. 'HoRsTMAN ETAL 2,354,123

INSULATION FOR SILICON IRONS Filed Aug. 16, 1941 2 Sheets-Sheet l wmv WWN

WITNESSES' INVENToRs Clifford C. Harstman v July 18, 1944- c. c. HORSTMAN. ErAL f 2,354,123

INSULATION FOR SILICON IRONS Filed Aug. 16, 1941 2 Sheets-Sheet 2 Resistance in .Ohmsper Sqm a 2 R3 Cliffrd C'Harstmn d:

Wel on H raadt.

. r PatentedA 1944Y i um'rso ls'rlrrss PATENT. orales I Y msULA'rroNzrsosltlszlJcoN mons v v Clifford C. Horstman, Sharpsville, and Weldon H. Brandt, Pittsburgh, Pa., assignors to Westinghouse Electric a Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 16, 1941, Serial No. 407,200 s claims. (ci. 14847) This invention relates to providing an insulating film on silicon iron in sheet form, primarily for use in electrical apparatus. Y

The object of this invention is to provide an insulating iilm on sheets of silicon-iron.

A further object of the invention is to provide a tenaciously adhering insulating film on siliconiron sheet materials by applying magnesium compounds which cause a chemical reaction on the surface of the sheets during an annealing cycle.

Another object of the invention is to provide anV insulating film cnsilicon iron strip having sufilcient adherence to permit winding of the strip into cores and capable of withstanding strain annealing temperatures.

Other objects of the invention will, in part', be obvious, Vand will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

Figure 1 is a diagram showing schematically how silicon-iron sheets or strip may be treated in accordance with this invention Fig. 1A is a schematic diagram in a continu- I ation of Fig. l showing how a core made from the silicon-iron sheet or strip may be treated and Worked.

Fig. 2 is a sectional view of a plurality of sheets of silicon-iron separated after a refractory coating after chemical reaction. i

Fig. 3 is an elevational view of a C-clamp spot testing device.

Fig. 4 isa graph showing the ohmic resistance plotted against heat treating temperature; and

Fig. 5 is a graph showing ohmic resistance of the nlm against different Awidths of laminations.

In recent years, an improved silicon-iron suitable for laminated magnetic cores has been' prepared by processes in which the crystals or grains of the silicon iron are aligned with their axes in substantially the same direction. Such material is known to the art as preferentially oriented silicon-iron. The material is characterized by a high permeability in one direction referred to as the direction of easiest magnetization, generally in the direction of rolling,'and the losses of the material are exceedingly low for all alternating eid densities in this direction. The magnetic properties of oriented sheets in the direction of Y easiest magnetization are greatly superior to randomly oriented silicon-irons.

When silicon-ironwith preferred grain orienta-- tion is subjected to alternating magnetic fields in directions other than that of the easiest magnetization. the permeability is much lower and Ythe losses are greater than that of unoriented magnetic material.

Because of the directional magnetic characteristics in the oriented silicon-iron strip, there are not the advantages in employing the strip in laminated cores when L-plate punchings are employed that may be obtained in continuously wound cores. In L-plate punchings, for example. the magnetic current may flow a portion of its path through material having low losses and high permeability and for the remainder of its path it flows through material having high losses and low permeability.

In order to take advantage of the good properties of grain oriented silicon-iron, it is good practice to wind the strip into circular or oval cores with the winding being in the direction of grain orientation. In such manners cores may be produced in which the magnetic currents induced by alternating fields are in the direction of easiest magnetization and full advantage is taken of the best characteristics of the material.

Experimentallyit has been found that wound cores made in this manner may be produced with losses comparable to the losses in same material found by Epstein tests. Furthermore, due to the shape of the core made by continuously winding a strip, it is possible to produce a magnetic core with a good space factor resulting in the eflicient use of silicon-iron and copper.

In the building `of wound core transformers having high electrical efilciency, the laminations of the core should 'carry an-adequate electrically insulating film. The insulating film in particular must have a high ohmic resistance, it must adhere tenaciously to the turns of the magnetic core, and must withstand thermal and mechanical stresses developed in the laminations both when building the apparatus and operating the apparatus. Furthermore, in order to meet design and manufacturing considerations, the thickness of the film on the turns of the core should be such as to permit of a space factor of the order of 90%, preferably 94% or 95%.

While it would be possible to insulate core 1aminations from each other by introducing a large amount of known insulating materials between laminations to give low eddy-current losses, the space factor considerations impose a requirement whichis difficult to meet by any known insulating material that will also meet other requirements of this type of structure, namely, tenacious adherence to laminations and capacity to with- .standing thermal and mechanical stresses developed during processing.

By means of this invention an insulating film l having characteristics which will meet the above requirements is produced. Brieiiy, by following the process herein disclosed, an insulating film composed of a complex glass-like reaction product of iron oxide, magnesia and silica is formed directly and integrally on the surface of the laminations of magnetic material during a heat treatment to develop the optimum magnetic and low loss characteristics in the core material. 'Ihe exact chemical composition of this glass-like complex of iron oxide-magnesio. and silica is not fully known at the present time. By microscopic and chemical analysis it is known that the lm includes particles of the refractory, such as magnesia, -bonded to the siliceous glass. By following the method of producing it, as given herein in detail, this film with its predetermined characteristics may be secured consistenly regardless of a lack of a complete knowledge of its chemical or physical composition.

There are certain requirements which will be imposed upon the film in the particular wound core building operations which will be hereinafter disclosed. Specifically, these considerations require a tenacious adherence of the film to the strip magnetic material to permit the strip to be bent about a radius of approximately one-eighth inch without rupture of the film. Further, to meet manufacturing requirements, the strip of material should have a film which is relatively dust-free and will not scale or break up during manipulation. In addition, the film must have such properties that the magnetic material may be exposed to strain annealing temperatures of the order of 900 C. without the film fusing or deteriorating.

The glassy insulating lm composed of iron oxide-magnesia and silica of this invention adequately meets the above requirements as called for by the wound core construction that will be detailed in the present application. In addition, the film is suitable for application to magnetic material which is to be assembled into other types of cores having less exacting mechanical andx electrical requirements imposed thereon.

A number of methods of producing preferred grain orientation are known in the art. Usually the processes embody a final cold rolling of the silicon-iron to a predetermined thickness- 13 mils being a common thickness for sheet to be employed in electrical apparatus. The cold rolled sheets are produced with a low carbon content of the order of 0.015%. A portion of the siliconiron in the cold rolled sheets is distributed in the form of grains having preferred orientation which will serve as nuclei for rearranging sub-V stantially all of the remainder of the silicon-iron into grains-having the desired orientation.

The cold rolled sheets are subjected to annealing in a temperature range of 900 C. to 1300 C.

in order to cooperate in the grain orientation. In addition the annealing treatment is conducted in an atmosphere which will assist in decarburizing the silicon-iron to reduce the carbon content l is coated with a refractory of some sort t'o prevent welding of the turns or sheets. i

It has been discovered that a selected refractory composition applied to the sheets of siliconiron in the manner herein disclosed will accomplish a dual function. First, the refractory will operate to prevent sticking of the sheets during the heat treatment. Second, under predetermined conditions, the refractory will cooperate in a chemical reaction on the Surface of the silicon-iron to produce a glassy insulating nlm having highly desirable characteristics.

In the specification and claims, the term stacking is intended to denote both the rolling of strips to provide a plurality of turnsand the piling of sheets into a large stack preparatory to annealing.

Referring to Fig. 1 of the drawings, a schematic showing of the sequence of operations required to produce heat treated silicon-iron sheet with an insulating film thereon is shown. Long strips of sheet material as well as separate sheets may be subjected to the same treatment.

Sheets of oriented silicon-iron, such as are produced by known methods are introduced into the furnace l0 having a decarburizing-oxidizlng atmosphere, such as air, or other gases, in order to impart a slight surface oxidation. It is preferable to pass the sheets singly through the furnace operating in a temperature range of approximately 500 C.600 C. for a brief time, in order to impart a blue or blue-black temper film to the sheets or strip. vThese blue temper films are of the order of 0.00001 inch in thickness. A blue or blue-black temper lm has been found to produce the most satisfactory results in practicing this invention. Y

A blue-black color or temper film on the surface of the sheets will be produced in from 2 to 6 minutes at about 500 C. to 600 C. Of course, by properly relating the time and temperature at which the oxidation takes place, the preoxidized surface may be provided at higher temperatures in shorter time. Some benefit may be obtained by oxidizing at lower temperatures, but generally this takes too much time.

It is important not to over-expose the sheets of material to the oxidizing gases, inasnuch as excess oxidation results in a poor magnetic sheet having high losses and also non-uniform lm' formation. Preoxidation, such as 10 minutes or longer, at 700 C., in air, produces excessive iron oxide on the surface of the sheet. During subsequent processing, this large quantity of iron oxide will be reduced to globules of iron which will tend to short circuit the silicon iron sheets in a core and thereby tend to increase the eddy current loss. v

Following preoxidation, the sheets of siliconiron are cooled. After cooling, the sheets of silicon-iron are coated with a refractory material such as magnesium oxide suspended in a suitable liquid. 'Ihe coating will function to prevent sticking of the sheets during the heat treatment at elevated temperatures and also to chemically react with the preoxidized surface to produce' a thin glassy insulating film.

The sheets of material may be coated with the refractory suspension in the apparatus Il of Fig. l. The coating apparatus consists of two rolls I2 of suitable configuration to apply a thin uniform coating of refractory suspension. The two rolls l2 operate respectively on the upper and lower surfaces of the sheet. 'Ihe lower roll I! is slightly immersed within a trough Il containing the refractory suspension I6, while the upper roll I2 is fed with -a thin stream I O of refractory suspension from'the container 20. No attempt is made to vimpart an absolutely uniform thickness of refractory suspension by means of the roll I2, but for this purpose two squeeze rolls 22 are employed.- The squeeze rolls are composed of soft rubber and are journalled within a framework capable of very sensitive adjustment in pressure in order to control the thickness of the refractory coating upon each of the facessof the sheets. The squeeze rolls 22 are preferably groove'd for best results. The excess coating is run back into trough Il. g

Other methods of applying the composition to the sheets of silicon iron may be employed. Dip- Y ping the sheets into a bath of the composition or y the market several types of magnesium oxide v graded on their physical and chemical char-acteristics. An extremely fine powder with good purity is that' which is known as U. S. P. extra light. This material has characteristics of being readily hydrated in water. Other available magnesium oxides suitable for use in this invention are not as finely pulverized and their chemical composition indicates the presence of a relatively large,amount of oxides other than magnesia. The following table gives the approximate analysis of three magnesium oxides available on the market:

.Constituent U' Sii Extra A B MgO 90. 2 89. 76 84. 45 C30 0. 28 2. 78 1. 25

' o. 02 .0i 0. l

H 7. 3 3. 67 s. 22 2. 1 i-. 13 5. o8 0. 10 2. 65 0. as

For the purpose of this invention, the U. S. P. extra light magnesium .oxide has certain desirable characteristics in that it forms a thick liy- 'drate or suspension quite rapidly when agitated with the proper amount of water. This suspension has very good adhesive characteristics when applied to thestrips of preoxidized sheet material. In many instances it has been found, however, that the U. S. P. extra light magnesia is low in silica, and under such conditions in order to produce consistently satisfactory complex magnesium silicate films on the sheets of magnetic material during the heat treatment operations, it may be advisable .to add silica. In many films have been produced with both A and B without silica additions. In some instances, however, the addition of from 2% to 5% of silica flour to composition B have resulted in a better film being produced thereby.

When the term silica is used in the claims in describing the coating or coating composition, it is intended tov include silicon dioxide,

either as additions to magnesia or as naturally present in the magnesia as an impurity.

A convenient method oi' producing a suspension of magnesia, with or without silica additions, is as follows: Approximately 500 pounds of the finely pulverized magnesium powder is placed within a large agitator containing- 650 gallons of water. The agitator is put into operation for one-half hour in order to cause a thorough mixing. The thoroughly agitated mixture is then allowed to stand for a period of from three to fifteen hours or longer in order to thoroughly hydrate the magnesium oxide. U. S. P. extra light magnesia. will hydrate in three hours, the other compositions will require a longer time. At the end of this period, several hundred gallons more of water are added in order to make 1000 gallons of suspension. This additional amount of water is blended with the hydrate by agitating for an additional one-half hour.

'I'he resulting material is 'a rather thick cream which contains from 'l1/2% to 8% hydroxide. It has been found that from 4% to 8% of hydroxide in the suspension produces the best coatings. Less than 4% of'hydrated magnesia results in a thin watery material which readily runs ofi the sheets and is without adequate adhesion. If over 8% of hydrated magnesia is present, the material is generally too thick to be applied in satisfactory coatings. In some cases, a slight amount of ball milling of the magnesium oxide and the silica flour in water will cause a better suspension of the silica fiour in the hydrated magnesia as compared to simply agitating the two in water.

In using'the U. S. P. extra light magnesia with approximately 5% silica flour addition thereto, it has been found that approximately 10 pounds of the 5% magnesium oxide suspension applied to a ton of 13 vmil sheet will give a coating productive of the best results. When less than 5% silica is present in the U. S. P. magnesia, for example, approximately 2%, then up to 20 pounds of magnesia per ton of 13 mil sheet should be applied as a coating to produce a good insulating film.

In using the magnesia of composition A or B, due to its relatively coarser structure and less satisfactory hydration which appears to be a me- .chanical suspension rather than a hydrate, it

has been found that approximately 20 to 30 pounds per ton of compositions A and B may be required per ton of 13 mil sheet .being coated to produce a good insulating film. This amount of magnesia is several times that required solely to prevent sticking.

The strips of silicon iron, after being coated with a suicient amount of suspension and squeeze rolled, are dried in an oven 2l at a temperature of below 300 C. It is desirable to stay below 300 C. in drying the coating on the sheets lof `preoxidized silicon-iron, inasmuch as the iron may' oxidize excessively at temperatures above 300 C. in the presence of large amounts of moisture. The purpose of the drying treatment is to cause both evaporation of the free water in the coating, and in addition it is believed to causeA at least a part of the magnesium hydroxide on the sheets to dehydrate to an oxide. Temperatures of from 200 C. to about 300 C. have been found satisfactory. 'I'he drying should be conducted slowly in order to prevent heavy coatings from fiaking. Slow drying will result in these heavy coatings tightly adhering to the sheets without ,loss of material. The coated strip,

of magnesium oxide with some magnesium hydroxide present which adheres satisfactorily for further; operations on the material.

The Astrip of silicon-iron with dried coat oi.' magnesiol is then subjected to a heat vtreatment within furnace 26 within a temperature range of from 900 C. to 1300 C. in order to produce grain growth and to convert the material into the preferred grain orientation. For best results, it has been found that furnace 26 should have a reducing and decarburizing atmosphereprefer ably hydrogen gas with a dew point of approximately 25 C. or less. The period for the heat treatment is from 1 to 30 hours, depending on the temperature and the size of the stack or coils of the material. At 1300 C., satisfactory orientation of all the material in the sheets may be obtained within a. few hours at the most. At 900 C., a longer heat treatment time is required. Due to the length of time required by the heat treatment, sheets of material are generally stacked in masses weighing several tons. Where a continuous strip of material has beensubjected to the preceding operations, it is rolled into a large cylindrical coil and one or more are generally placed on end withinthe annealing furnace.

Referring to Fig. 2 of the drawings, an enlarged cross-section of several sheets 60 of silicon-iron, are shown stacked in the annealing furnace. The coating S4 between the sheets of silicon-iron comprises magnesium oxide plus 'any additions of silicon oxide. The coating 64, when subjected to temperatures of from 900 C. to 1300 C. reacts with the oxidized surface of the silicon-iron sheets, and some of the silicon in the silicon-iron alloy, to produce a fluid of complex ferro-magnesium silicate. Silicon in the sheet adjacent the preoxidized surface is preoxidized to silica along with the iron and diffuses to the surface of the sheets.

From experimental tests conducted, it has been found that, in the absence of the oxidized metal surface such as is produced -in furnace I0, substantially no glass film-producing reaction occurs between the coating 64 and the sheet 60. It has been found necessary to secure, in one way or another, an initial thin film of metallic surface oxides in order to produce the insulating silicate glass film.' It is, accordingly, believed'that the film, such as is indicated at 62 in Fig. 2, is the combined reaction product of magnesium oxidesilica and iron oxide.

characteristics ofthe complex glass nlm. Below 1150 C., the film does not form rapidly and it tends to be quite thin and fragile. At 1150 C.,

or slightly above, the reaction between the iron The nature and exact chemical composition of the glass is not known. Examination of crosssections of the `film on silicon-iron under polarized light shows the presence of magnesia particles bonded to each other and to the iron by a glassy film which it is believed consists of a ferro-magnesium silicate. In some instances, the insulating film includes a large proportion of particles of magnesia. The electrical Yinsulation is improved by the presence of the magnesia particles in the glass film.

Chemical analysis of the glassy lm is quite difficult due to its extreme thinness and adherence to the silicon iron. Films on sheets which have been brushed free of loose magnesia particles and the film detached from the silicon iron have been analyzed and showed that silica and magnesia were combined in approximately a one-to-one ratio. The films contained from 1.4% to 5.7% iron.

The temperature has been found to exert a critical effect on the formation and physical oxide,magnesia and silica proceeds with greater speed. The glass-like film is thicker. In most instances the lm produced at 1150' C. has good electrical resistance. The adhesion of the magnesia particles enhances the electricalv insulating characteristics. It is therefore advisable not to brush film produced at this temperature severely and dislodge the bonded magnesia particles.

At 1200 C. and higher, the chemical reaction of iron oxide, magnesia and silica is quite rapid and more complete than at lower temperatures. The best type of insulation, mechanically and electrically, is secured at these temperatures. The film is considerably heavier and more durable than those produced at 1150 C. or lower.

Using the extra light magnesia with 5% silica addition, film having the following characteristics was produced at several temperatures:

Median resistivity Temperature, C.

ohms/cm.'

Hic

While the film is not entirely a homogeneous l glass but has particles of magnesia embedded and 2 bonded to the glass, it may be brushed thoroughly to remove the unattached magnesia particles in the loose layer 64 without impairing its insulating characteristics. The sheet with this film may be bent about radii' of one-fourth inch without v causing separation. The sheet may be rolled vigorously in subsequent core winding operations without causing noticeable loss of the film or decrease of insulating value.

Under the conditions established by the precedingfoperations, the glass film is limited to a narrow zone on the surfaces of the silicon iron sheets. The film extends as a glassy phase approximately 0.1 mil or less outwardly from the silicon iron surface. Under the same heat treatment temperatures, the remainder of the coating 64 proper, which is out of contact with the preoxidized metal surface, does not change to a fluid, and does not fuse. It operates satisfactorily to prevent welding After a heat treatment within furnace 26 sum-l cient to produce the preferred grain orientation' and optimum magnetic properties in the silicon iron, the material is cooled to below 500 C. in the furnace before .the sheets are exposed to the atmosphere. After completely coolingthe sheets ascuas are subjected to a brushing operation by means of wet or dry brushes. such as 28. This operation brushes away all of the loose coating 84 and any scale or non-adhering material. 'I'his brushing should not be so severe as to loosenthe illm or shatter it. The individual sheets have a gray appearance due to the presence of a thin illm of insulating silicate 82.

In order to determine whether a satisfactory film has been produced, the fllm. may be tested directly for its electrical resistance. It has been found that a satisfactory criterion of the quality ofthe illm is a median resistance of 1 ohm or more per square centimeter at a contact pressure of'approximately 50 pounds per square inch when the measuring contacts are given a slight twist` 3. The apparatus 10 is knownas the C-clamp spot tester and has been developed specifically for use in determining whether or not the lm 62 on sheet 60 is of a quality suitable for making wound cores.

The C-clamp spot tester 10 consists of a rigid U-frame 12 of a depth suiicient to accommodate the width of the sheet being tested. At the open' end of the U-frame 12 is a fixed-contact member 14 arranged on an insulated support. A movable contact member 16 is disposed above the fixed contact. Both contact members 14 and 16 are of the area of one square centimeter for the purposes of the tests' in question. The movable contact member 16 is carried at the extremity of an axially movable shaft 18 which is spring biased by the spring 80 into contact with 14. The spring 60 is of a strength \to apply a force of 50 pounds per square inch. In separating contactmember 16 from contact member 14, a lever arm 82 connected to shaft 18 is employed. Conductors 84 and 86 are connected to the contact members 16 and 14, respectively.` Conductors 88, in circuit with conductors 84 and 86, connect the contact members with a suitable instrument 90, such as an ohmmeter and the like, for measuring the resistance of any material placed between the contact members 14 and 16.

In operation, the C-clamp spot tester 10 is applied to a sheet of material whose resistance is to be determined by separating contact 16 from 14 by proper manipulation of the lever 82. When the contacts are disposed over the area whose resistance is to be measured, the lever arm 82 is released and the spring 80 will cause the contact members 16 and 14 to clamp the material on opposite surfaces with a force of 50 pounds per square inch. It is preferred to give the U-frame 12 a slight twist of from 10 to 90 prior to taking readings onohmmeter or other resistance measuring device 80.

Y PreviouslyL the accepted or standard tests for measuring the insulation on laminations consisted in super-imposingv a number of laminations of given size and testing the overall resistance and dividing the value by the number of laminations. In a great number of experiments, it has been found that such prior art test for insulation on magnetic material did not correlate with material which produced satisfactory wound cores 34. Apparently, these prior art tests of the nlm resistance measured undisturbed nlm resistance and did not nlm when disturbed or abraded during winding into cores.

I'he C-clamp spot tester 10 is employed in testing the insulating value of nlm on single sheets. The spot tester has the advantage over the prior art testing equipment in that it may be used on single sheets and the tester may be employed to determine the value of the insulation `of any given area. As is well known to those nealed and filmed material is suitable for producing wound cores in that the tester 10 determines the combined electrical resistance and mechanical strength of the lm when it is applied with a twisting motion to 'the particular area being tested. The twist under pressure will cause fragile film of a type which would fail in winding of cores to abode or shatter and there-f` .fore such fragile material will not indicate adequate electrical resistance. f

Since the strips of silicon iron are generally produced in widths of 24 inches and most transformer cores are of much narrower widths than this, the strip must be slit to required width. This is effected by the slitting rolls 30 of suitable design. The slit siliconiron is then formed into a Wound core which it has been found makes use 'of the advantages of the oriented sheet t0 the maximum extent. The strips are wound on a mandrel 32 of desired configuration. To secure the predetermined space factor of over the strip is wound into a core 34 of suitable thickness under the application of pressure from a roll 35. The insulating llxn obtained by the method of this invention and passing the C-clamp test may be subjected to' this winding operation D without substantially cracking or failing. Very small radii of one-eighth inch at the corners of the mandrel may be employed.

The wound core 34 is strain annealed in a furnace 36 at a temperature of from 700 C. to 1000" C. or even higher in order to remove the strains induced by the winding operation. Very satisfactory strain annealing results are obtained in the range' of 850 C. to 900 C. The insulating film obtained by the process herein described is not impaired by the strain annealing operation. 'I'he film does not melt or soften at these temperatures. I

In order to most conveniently and eiliciently embody the wound core 34 into a transformer. construction, it has been found desirable to split the wound core into two parts. For the purpose of splitting the core without loosening the laminations from the wound cnstruction,- the sequence of operations, shown in Figure 1A, is followed.

After strain annealing, the wound core is impregnated with a resinous bonding material capable of filling allthe spaces between and consolidating the laminations into a solid core. The impregnation treatment is conducted within the impregnating tank 38 of any suitable design. For best results, vacuum impregnation may be used. The resinous material retained between laminations is then caused to solidify within the take into account the resistance properties of the 15 heat treating apparatus 48.

The core 34, after the impregnating and resin heat treating operation, is a substantially solid body which may be cut into sections without delamination. The resin is keyed to the insulating film and working stresses will be absorbed by the nlm without causing rupture. The bonded magnetic core 34 is placed in the holding member 44 where it may be sawed into predetermined sections by the metal cutting saw 42. The sections may now be readily assembled by slipping them into suitably shaped coils and combined with other electrical elements into a transformer proper.

It has been found that the sawing operation produces a great number of slivers and rough burrs between laminations which short circuit the laminations at the cut joint. Furthermore, the cut face or joint may be rough and introduce a high air gap loss into the magnetic core. For this reason, the cut surfaces are subjected to an additional series of operations including a grinding operation by means of a grinding wheel 46. For grinding, the sections of the core 36 are installed within a jig 48 having a clamping block 50 and clamping screws 52 in order that the surfaces produced by the grinding wheel may be both parallel and at.

In some instances, it has been found that an additional etching operation is required to eliminate the finest burrs or slivers which have been untouched by the grinding wheel.

The two sections of each core with flat, parallel cut joints produced by the grinding and etching operation may be recombined in the final transformer, such as shown at 54, with very low air gap loss between the several sections. It will be appreciated that the grinding and cutting operations should not cause failure of the insulating film produced prior thereto, and the resin should not cause detachment of the film under the working stresses. Since the insulating film of complex ferromagnesium silicates adheres eX- tremely tenaciously to the silicon iron, this sequence of operations is performed with negligible delamination or core damage.

In manufacturing transformers and other electrical apparatus, it is a lcustomary requirement that the eddy current losses in a magnetic core should not exceed 1% of the total loss. It is known thai; the eddy current loss of a magnetic core may vary with the thickness of the laminations, the width of the laminations and the interlamination resistance. Therefore, for a given thiclmess of material, the interlamination resistance should increase with increase in Width of the material being made into a core in order to keep the eddy current losses below a predetermined value.

Referring to Fig. of the drawings, there is plotted a curve of the resistance in ohms per square centimeter required for various Widths of material in order to give eddy current losses of 1% of a total loss of 0.86 watt per pound for 13 mil sheet. It Will be seen from an inspection of Fig. 5 that 2 inch wide material requires an interlamination resistance of 0.8 ohm per square centimeter to produce a core having an eddy current loss of 1% of .86 Watt per `pound of silicon iron. When the width of the core material is '7 inches, a surface resistance of 10 ohms is necessary to reduce the eddy current loss to 1% of 0.86 watt per pound.

It has been previously specified that insulating film having a resistance of one ohm or more per square centimeter as4 measured by C-clamp tester is satisfactory for producing highly emcient wound transformer cores. In practice when 'wide sheets 6 inches and wider are wound into cores more than one ohm resistance per square centimeter is required. However the C-clamp test is so severe that it has been found that film which tests one ohm or more per square centimeter is satisfactory for producing cores of greater width than 21/4 inches.

It will be appreciated that in testing any given sheet of material carrying glassy film thereon as produced in the process described herein, there will be a variation in the resistance of the glassy film from point to point. In selecting material carrying a sufficient insulation to meet any predetermined requirement, satisfactory material is that which has a median resistance of one ohm or more per square centimeter, that is, as many points will register above this value as below. When such material is wound into a core, the distribution of the high and low insulation areas ordinarily will be sufficient to effect satisfactory insulation. However, the areas having high electrical insulation and those having low electrical insulation should be relatively small in order that in winding they will effect satisfactory insulation.

From the preceding description, it will be seen that the complex magnesium silicate-iron oxide film meets the requirements of good electrical resistance, tenacious adherence to the iron, and resistance to the effects of temperature and mechanical deformation. The method of producing this insulating film is so controlled that the film can exist concurrently as a fused phase with the infusible phase in the refractory coating during annealing. By means of the predetermined preoxidation treatment, the thickness and extent of this film are controlled to a high degree. A good space factor is secured by the fact that this film has a thickness of about 0.0001 inch or less.

' Since the insulating film is produced concurrently with and as a byproduct of the annealing process, which requires a refractory to prevent sticking, it is low in cost.

- Since certain changes in carrying out the above process and certain modifications in the article which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. Therefore, itis desired that the invention be interpreted as broadly as possible and that it be limited only by what is expressly set forth in the following claims.

We claim as our invention:

1. The method of treating sheets of siliconiron to product an electrically insulating lrn on the surfaces of the sheets during an annealing process to develop grain orientation, which comprises, in combination, preoxidizing the sheets to produce a thin blue temper iilm of oxide on the surfaces of the sheets, coating the sheets with magnesium oxide and silica capable of chemically reacting with the oxide temper film at annealing temperatures, stacking -a plurality of the coated, preoxidized sheets and heat-treating the stacked sheets at a temperature of from 1l50 C. to 1300 C. in a reducing atmosphere comprising hydrogen gas for a period of time suflicient to develop grain orientation and to provide for the chemical reaction of the oxide and the magnesium oxide coating at the surfaces of the sheets to forml an electrically insulated film which adheres tenaciously to the sheets.

2. The method of treating sheets of low carbon silicon-iron to substantially decarburize the sheets and to produce an electrically insulating film on the surfaces of the sheets during an annealing process to develop grain orientation which comprises, in combination, preoxidizing the sheets of silicon-iron to form a blue temper oxide film on the surfaces, applying magnesium oxide and silica to the sheets, and heat treating the sheets in. an atmosphere comprising hydrogen gas at a temperature of from 11 50 C. to 1300 C. to decarburize the silicon-iron and to provide for a chemical reaction of the oxide film, the silica and the magnesium oxide to form a tightly adherent insulating film.

3. In a method of treating sheets of cold rolled, low-carbon silicon iron to substantially decarburize the sheets in annealing of the silicon iron in the process of developing grain orientation and to provide an electrically insulating film on the surfaces of the sheets, which comprises, in combination, peroxidizing the sheets of silicon-iron to form a film of oxide having a blue temper color on the surfaces, coating the surfaces of the sheets with the equivalent of from to 30 pounds of magnesium oxide carrying a minor proportion of silica per ton of 13 mil sheets and heat treating the preoxidized and coated sheets in a decarburizing and reducing atmosphere at a tempera? ture of 1100 C. and higher to substantially decarburize the sheets and to cause a chemical reaction of the fllm of oxide, the silica and the magnesium oxideto produce a tightly adherent film on the sheet surfaces.

4. In a method of treating sheets of cold rolled silicon-iron having a low carbon content to substantially decarburize the sheets in the anneal- {ing of the silicon-iron in the process of developing grain orientation and to provide an electrically insulating fllm on the surfaces of the sheets which comprises, in combination, preoxidizing the surfaces of the sheets to form a film of oxide having a blue temper color, applying to the surfaces of the sheets the equivalent of from 1,0 to 30 pounds or more of magnesium oxide carrying a minor proportion of silica per ton of 13 mil sheets, stacking the sheets and heat treating the stacked sheets in a decarburizing and reducing atmosphere comprising hydrogen at a temperature of 1100 C. and higher to substantially decarburize the sheets, and to cause the film of oxide to chemically react with a part of the magnesium oxide and the silica to produce a tightly adherent a electrically insulating film on the surfaces of the sheets, particles of unreacted magnesia being attached to the film, while the remainder of the magnesium oxide remains unreacted to s erve to prevent sticking of the stacked sheets.

5. In a method of treating sheets of siliconiron to produce anelectrically insulating nlm tightly adherent tothe sheets in the annealing of the silicon iron in the process of developing grain orientation which comprises, in combination, preoxidizing the silicon iron sheets to a blue temper film oxide coating, applying magnesium oxide and a minor proportionof sil-ica to the oxildized sheetsand heat treating the sheets in an iron having surfaces carrying a blue temper oxide film, which comprises applying to the surface of the sheet as a coating, a composition composed of magnesium oxide and a minor proportion of silica, subjecting the coated silicon iron to a heat treatment above 900" C. to effect a reaction of the ingredients of the coating composition and the blue temper oxide film on th'e silicon-iron to produce an adherent electrically insulating film on the surface ofthe silicon-iron. Y

'7. A cold rolled silicon-iron strip having a preferred grain orientation and an electrically insulating glass lm tightly adherent to the silicon iron strip, theglass film comprising essentially -the reaction' products of a layer of iron oxide of about the thickness of a blue temper nlm, and a composition having from about 1% to 10% of silica and the balance substantially magnesium oxide at elevated temperatures of 1100% C. and higher, the film being capable of withstanding a strain anneal of the strip at temperatures of the order of 900 C. and so adherent that under a C- clamp test it-will retain a median electrical resistance of the order of one ohm per square centimeter of strip surface for strip about two inches in width and permit winding of the strip on arcs of the order of A inch diameter without substantial rupture ofthe film.

8. A cold rolled silicon-iron strip having a preferred grain orientation and an electrically insulating glass film tightly adherent to the siliconiron strip, the glass film comprising essentially the reaction products of a layer of iron oxide of about the thickness of a blue temperfilm, and a composition having from about-1% to 10% of-silica and the balance substantially magnesium oxide at elevated temperatures of 1100 C. and

higher, and unreacte'd particles of magnesia adhering to the glass film, the film being capable of withstanding a strain anneal of the strip at temperatures of the order of 900 C. and so adherent that under a C-clamp test it will retain a median electrical resistance of the order of one ohm per square centimeter of strip surface for strip about two inches 1n width and permit winding of .the 

